The spine is formed by superposed vertebrae, that are normally in alignment along a vertical axis, from the lumbar vertebrae to the cervical vertebrae, each vertebra presenting a posterior wall from which there projects a spinous process and presenting two sides having walls from which there project ribs and/or transverse processes. When the spine of an individual presents abnormal curvature, the vertebrae are inclined relative to one another and relative to said vertebral axis. The sides of the vertebrae situated on one side are thus moved towards each other so as to form a concave side, whereas the sides of the vertebrae on the other side are moved apart from each other so as to form a convex side.
In order to straighten the vertebral column, the sides of the vertebrae on the concave side need to be moved further apart so as to be situated relative to one another at a distance that is substantially equivalent to the distance between the sides of the vertebrae on the other side. In order subsequently to hold the vertebrae relative to one another, known devices have screws that are inserted into the vertebrae or hooks that are inserted along the inside wall of the spinal canal and rods for interconnecting the screws or the hooks.
The hooks are generally inserted in pairs into each vertebra and from either side close to the pedicles, their heads projecting from the posterior wall of the vertebra, one on either side of the spinous process. The heads form sockets, for example, suitable for receiving a rod which is locked in position by means of a nut screwed onto the head and bearing against the rod. The rows constituted by the heads of the hooks situated on either side of the spinous processes are interconnected and held in a fixed position by two rods that are parallel to each other and to the axis of the spine.
Nevertheless, hooks are difficult to use since the operator must under no circumstances harm the spinal cord which occupies the center of the spinal canal, since otherwise the patient will be paralyzed.
The use of screws makes it possible to reduce the risks of such surgery. The screws likewise present socket-forming heads and are inserted in pairs into the posterior walls of the vertebrae in the pedicles on either side of the spinous processes. Thus, the screws constitute fixing points in the vertebrae for holding them relative to one another. Nevertheless, they are necessarily inserted into the pedicles of the vertebrae which can, in some circumstances, be small in size or easily damaged.